A. Field of the Invention
The invention relates to a torque converter, and particularly to a torque converter provided having a lockup device that operates in response to change in hydraulic pressure within portions of the torque converter.
B. Description of the Background Art
A torque converter is a device that includes three kinds of vane wheels, an impeller, a turbine and a stator, which together provide a means for transmitting torque via an internal working fluid. The impeller is fixed to a front cover of the torque converter, the front cover being configured to receive inputted torque. The impeller includes an impeller shell which together with the front cover surrounds and defines a fluid chamber filled with the working fluid. The turbine is located within the fluid working chamber And faces the impeller. A main drive shaft of a transmission is connectable to the turbine. When the impeller rotates, the working fluid flows from impeller blades of the impeller shell toward the turbine, thereby causing the turbine to rotate. Consequently, the turbine transmits torque to the main drive shaft of the transmission.
A lockup device is a mechanism, which is disposed in a space between the front cover and the turbine for mechanically coupling the front cover to the turbine and thereby directly transmitting the torque therebetween. The lockup device typically includes a piston and an elastic coupling mechanism for coupling the piston to an output member such as a turbine. The piston is, e.g., a circular plate-like member, and divides the space between the front cover and the turbine into a first hydraulic chamber adjacent to the front cover and a second hydraulic chamber adjacent to the turbine. As a result, the piston can move toward and away from the front cover in accordance with changes in hydraulic pressure between the first and second hydraulic chambers. A friction engaging portion having a friction facing is formed on the front cover side of an outer peripheral portion of the front cover. When the working fluid in the first hydraulic chamber is drained to increase relatively the pressure in the second hydraulic chamber, the piston moves toward the front cover so that the friction facing is pressed against the friction surface of the front cover.
The elastic coupling mechanism is formed of, e.g., a drive member fixed to the piston, a driven member coupled to the turbine and elastic members such as coil springs arranged between the drive and driven members for transmitting torque and absorbing vibrations and fluctuations in the torque.
In order to engage the lock-up clutch, the working fluid in the first hydraulic chamber is drained, and the working fluid is allowed to flow into the second hydraulic chamber. As a result, the pressure in the second hydraulic chamber increases relative to the pressure in the first hydraulic chamber. As a result the piston moves toward the front cover.
However, under some operating conditions, as the working fluid is urged from the impeller blades toward the turbine, the fluid pressure in the torque converter proximate the radial outer edges of the turbine and impeller blades may be lower than the fluid pressure in the second hydraulic chamber. As a result the working fluid may be may be drawn out of the second hydraulic chamber into the turbine. Consequently, the pressure in the second hydraulic chamber may lowered to a value lower than or equal to that in the first hydraulic chamber. If the pressure in the second hydraulic chamber is reduced too much, the piston may not be moved toward the front cover, and the lockup clutch will not be engaged. Under some conditions, the fluid pressure in the second hydraulic chamber may be lowered but still be sufficient enough to cause engagement of the lockup clutch. However, the engagement time or response time for engagement of the lockup clutch may be increased, i.e. it may take a relatively long time for the lockup clutch to become engaged. Preferably, the response time for engagement of the lockup clutch should be as short a period of time as possible.